Aluminum silicon (AlSi) alloys are well known in the casting industry. Metallurgists are constantly searching for AlSi alloys having high strength and high ductility and that can be used to cast various parts at a relatively low cost. Herein is described an AlSi alloy for use in a lost foam casting process and a method for utilizing the same.
Most AlSi die casting alloys contain magnesium (Mg) to increase the strength of the alloy. However, the addition of Mg also decreases the ductility of the alloy. Further, during the die casting solidification process, Mg-containing AlSi alloys experience a surface film that forms on the outer surface of the molten cast object.
Since most aluminum alloys contain some Mg (generally less than 1% by weight), it is expected that the surface film that forms is MgO—Al2O3, known as “spinel”. During the beginning of the solidification process, the spinel initially protects the molten cast object from soldering with the die casting die. However, as the molten cast object continues to solidify, the moving molten metal stretches and breaks the spinel, exposing fresh aluminum that solders with the metal die. Basically, the iron (Fe) in the dies thermodynamically desires to dissolve into the iron-free aluminum. To decrease this thermodynamic driving force, the iron content of the aluminum alloy traditionally is increased. Thus, if the aluminum alloy already contains the iron it desires (with traditionally, a 1% by weight Fe addition), the aluminum alloy does not have the same desire to dissolve the iron atoms in the dies. Therefore, to prevent die soldering, AlSi alloys, and even Mg-containing AlSi alloys, traditionally contain iron to prevent soldering of the alloy to the die casting molds. Significantly, in the microstructure of such alloys, the iron occurs as elongated needle-like phase, the presence of which has been found to decrease the strength and ductility of AlSi alloys and increase microporosity.
The solidification range, which is a temperature range over which an alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider or greater the solidification range, the longer it will take an alloy to solidify at a given rate of cooling. During a hypoeutectic (i.e. containing <11.6% by weight Si) AlSi alloy's descent through the solidification range, aluminum dendrites are the first to form. As time elapses and the cooling process proceeds, the aluminum dendrites grow larger, eventually touch, and form a dendritic network. During this time frame, and sometimes even before the precipitation of the primary aluminum phase, the elongated iron needle-like phase also forms and tends to clog the narrow passageways of the aluminum dendritic network, restricting the flow of eutectic liquid. Such phenomena tends to increase the instance of microporosity in the final cast structure.
In general, tensile properties of any alloy decrease as the cooling rate decreases. This occurs because it is difficult to obtain a fine cast microstructure at very slow feeding rates, and because of the increased tendency for castings to be less sound if they freeze thoroughly, particularly if hydrogen porosity is not suppressed.
A high degree of microporosity is undesirable, particularly when the alloy is used for engine blocks, because high microporosity causes leakage under O-ring seals on machined head deck surfaces, and lowers the torque carrying capacity of machined threads. Further, hypoeutectic AlSi alloy engine blocks are designed to have electro-deposited material, such as chromium, on the cylinder bore surfaces for wear resistance. Microporosity prevents the adhesion of the electro-deposited chrome plating.
Similarly, AlSi alloys cast using a high pressure die casting method also result in a porous surface structure due to microporosity in the parent bore material that, if used in engine parts, is particularly detrimental because it contributes to high oil consumption. Conventionally, hypereutectic (i.e. containing >11.6% by weight Si) AlSi alloys have been used to produce engine blocks for outboard and stern drive motors in the recreation boating industry. Such alloys are advantageous for use in engine blocks as they provide a high tensile strength, high modulus, low coefficient of thermal expansion, and are resistant to wear.
Furthermore, microporosity in mechanical parts is detrimental because the microporosity decreases the overall ductility of the alloy. Microporosity has been found to decrease the ductility of a AlSi cast object, regardless of whether the object is cast from a hypoeutectic, hypereutectic, eutectic or modified eutectic AlSi alloy.
Nearly 70% of all cast aluminum products made in the United States are cast using the die casting process. As aforementioned, conventional AlSi die casting alloys contain approximately 1% by weight iron to avoid die soldering. However, the iron addition degrades mechanical properties, particularly the ductility of the alloy, and to a greater extent than any of the commercial alloying elements used with aluminum. As a result, die cast alloys are generally not recommended in an application where an alloy having high mechanical properties is required. Such applications that cannot traditionally be satisfied by the die casting process may be satisfied with much more expensive processes including the permanent mold casting process and the sand casting process. Accordingly, all AlSi die casting alloys registered with the Aluminum Association contain 1.2 to 2.0% iron by weight, including the Aluminum Association designations of: 343, 360, A360, 364, 369, 380, A380, B380, 383, 384, A384, 385, 413, A413, and C443.
Furthermore, experimentation has demonstrated that the tensile strength, percent elongation, and quality index of AlSi alloys decreases as the amount of iron increases. For example, an AlSi alloy having 10.8% by weight silicon and 0.29% by weight iron has a tensile strength of approximately 31,100 psi, a percent elongation of 14.0, and a quality index (i.e. static toughness) of 386 MPa. In contrast, an AlSi alloy having 10.1% by weight silicon and 1.13% by weight iron has a tensile strength of 24,500 psi, a percent elongation of 2.5, and a quality index of 229 MPa. In further contrast, an AlSi alloy having 10.2% by weight silicon and 2.08% by weight iron has a tensile strength of 11,200 psi, a percent elongation of 1.0, and a quality index of 77 MPa.
Therefore, it would be advantageous to reduce the iron content of die casting AlSi alloys so that the iron needle-like phases are reduced to facilitate interdendritic feeding and correspondingly reduce microporosity. However, it is also important to prevent die cast AlSi articles from soldering to die cast molds, a problem that is traditionally solved by adding iron to the alloy.
Additionally, AlSi alloys, and particularly hypoeutectic AlSi alloys, generally have poor ductility because of the large irregular shape of the acicular eutectic silicon phase, and because of the presence of the beta-(Fe, Al, Si) type needle-like phase. The aforementioned iron needles and acicular eutectic silicon clog the interdendritic passageway between the primary aluminum dendrites and hinder feeding late in the solidification event resulting in microporosity (as aforementioned) and also decrease mechanical properties such as ductility. It has been recognized that the growth of the eutectic silicon phase can be modified by the addition of small amounts of sodium (Na) or strontium (Sr), thereby increasing the ductility of the hypoeutectic AlSi alloy. Such modification further reduces microporosity as the smaller eutectic silicon phase structure facilitates interdendritic feeding.
U.S. Pat. No. 5,234,514 relates to a hypereutectic AlSi alloy having refined primary silicon and a modified eutectic. The '514 patent is directed to modifying the primary silicon phase and the silicon phase of the eutectic through the addition of phosphorus (P) and a grain refining substance. When this alloy is cooled from solid solution to a temperature beneath the liquidus temperature, the phosphorus acts in a conventional manner to precipitate aluminum phosphide particles, which serve as an active nucleant for primary silicon, thus producing smaller refined primary silicon particles having a size generally less than 30 microns. However, the '514 patent indicates that the same process could not be used with a hypereutectic AlSi alloy modified with P and Na or Sr, because the Na and Sr neutralize the phosphorous effect, and the iron content of the alloy still causes precipitation of the iron phase that hinders interdendritic feeding.
U.S. Pat. No. 6,267,829 is directed to a method of reducing the formation of primary platelet-shaped beta-phase in iron containing AlSi alloys, in particular Al—Si—Mn—Fe alloys. The '829 patent does not contemplate rapid cooling of the alloy and, thus, does not contemplate die casting of the alloy presented therein. The '829 patent requires the inclusion of either titanium (Ti) or zirconium (Zr) or barium (Ba) for grain refinement and either Sr, Na, or Barium (Ba) for eutectic silicon modification. The gist of the '829 patent is that the primary platelet-shaped beta-phase is suppressed by the formation of an Al8 Fe2 Si-type phase. Formation of the Al8 Fe2 Si-type phase requires the addition of Boron (B) to the melt because the Al8 Fe2 Si-type phase favors nucleation on mixed borides. Thus Ti or Zr and Sr, Na or Ba and B are essential elements to the '829 patent teachings, while Fe is an element continually present in all formulations in at least 0.4% by weight.
U.S. Pat. No. 6,364,970 is directed to a hypoeutectic aluminum-silicon alloy. The alloy according to the '970 patent contains an iron content of up to 0.15% by weight and a strontium refinement of 30 to 300 ppm (0.003 to 0.03% by weight). One of skill in the art understands that for this minimum amount of strontium to modify the eutectic silicon, it is absolutely imperative that phosphorus (P), which reacts with Sr and neutralizes it, must be present by less than 0.01% by weight. The hypoeutectic alloy of the '970 patent has a high fracture strength resulting from the refined eutectic silicon phase and resulting from the addition of Sr to the alloy. The alloy further contains 0.5 to 0.8% by weight manganese (Mn). Those of skill in the art will understand Mn is added to modify the iron phase to a “Chinese script” microstructure, and to prevent die soldering. The alloy disclosed in the '970 patent is known in the industry as Silafont 36. The Aluminum Handbook, Volume 1: Fundamentals and Materials. published by Aluminium-Verlag Marketing, & Kommunikation GmbH, 1999 at pp. 131 and 132 discusses the advantages and limitations of Silafont 36 and similar alloys: “ . . . ductility cannot be achieved with conventional casting alloys because of high residual Fe content. Thus new alloys such as AlMg5Si2Mn (Magsimal-59) and AlSigMgMnSr (Silafont 36) have been developed in which the Fe content is reduced to about 0.15%. In order to ensure there is no sticking [i.e. soldering], the Mn content has been increased to 0.5 to 0.8%, and this has the added, highly desirable effect of improving hot strength.”
During use, outboard marine propellers sometimes collide with underwater objects that damage the propellers. If the alloy that forms the propeller has low ductility, a propeller blade may fracture off if it collides with an underwater object of substantial size. High pressure die cast hypoeutectic AlSi alloys have seen limited use for marine propellers because they are brittle and lack ductility. Due to greater ductility, aluminum magnesium alloys are in general used for marine propellers. Aluminum magnesium alloys, such as AA 514, are advantageous as they provide high ductility and toughness. However, the repairability of such aluminum magnesium propellers is limited. The addition of magnesium to AlSi alloys has been found to increase the strength of propellers while decreasing the ductility. Thus, AlSi alloys containing magnesium are less desirable than the traditional aluminum magnesium alloys for propellers. Still, it has been found that aluminum magnesium alloys are significantly more expensive to die cast into propellers because the casting temperature is significantly higher and because the scrap rate is much greater.
For cost and geometrical tolerance reasons, propellers for outboard and stern drive motors are traditionally cast using high pressure die cast processes. Propellers may also be cast using a more expensive semi-solid metal (SSM) casting process. In the SSM process, an alloy is injected into a die at a suitable temperature in the semi-solid state, much the same way as in high pressure die casting. However, the viscosity is higher and the injection speed is much lower than in conventional pressure die casting, resulting in little or no turbulence during die filling. The reduction in turbulence creates a corresponding reduction in microporosity. Thus, it would be advantageous to be able to die cast, and particularly high-pressure die cast marine propellers.
Regardless of how marine propellers are cast, the propellers regularly fracture large segments of the propeller blades when they collide with underwater objects during operation. This is due to the brittleness of traditional propeller alloys, as discussed, above. As a result, the damaged propeller blades cannot be easily repaired as the missing segments are lost at the bottom of the body of water where the propeller was operated. Furthermore, the brittleness inherent in traditional die cast AlSi alloys prevents efficient restructuring of the propellers through hammering. Thus, it is desirable to provide a propeller that only bends, but does not break upon impact with an underwater object.
An outboard assembly consists of (from top to bottom, vertically) an engine, a drive shaft housing, a lower unit also called the gear case housing, and a horizontal propeller shaft, on which a propeller is mounted. This outboard assembly is attached to a boat transom of a boat by means of a swivel bracket. When the boat is traveling at high speeds, a safety concern is present if the lower unit collides with an underwater object. In this case, the swivel bracket and/or drive shaft housing may fail and allow the outboard assembly with its spinning propeller to enter the boat and cause serious injury to the boat's operator. Thus, it is a common safety requirement in the industry that an outboard assembly must pass two consecutive collisions with an underwater object at 40 mph and still be operational. Further, as the outboard assembly becomes more massive, this requirement becomes more difficult to meet. As a result, it is generally accepted that outboards having more than 225 HP have problems meeting industry requirements particularly if the drive shaft housings are die cast because of the low ductility and impact strengths associated with conventional die cast AlSi alloys. Accordingly, it would be highly advantageous to be able to die cast drive shaft housings with sufficient impact strength so that the drive shaft housings could be produced at a lower cost. Similarly, it would be advantageous to manufacture gear case housings and stern drive Gimbel rings for these same reasons.
In addition to the above, AlSi alloys are used in lost foam casting and lost foam casting with pressure processes to produce complex parts. However, parts manufactured using lost foam casting processes are traditionally brittle parts (such as blocks and heads) and has not been used to make damage tolerant, high impact resistant parts.
Currently, the industry standard for lost foam casting is aluminum alloy 356 (AA 356). Conventionally, during solidification, the hydrostatic tension in the liquid alloy builds up and fractures the surface skin. This fracture is due to the fact that the AA 356 alloy, with a silicon content of 6.5 to 7.5% by weight, has a relatively wide solidification range and therefore forms a relatively thin skin on the outer surface of the cast product during cooling. The skin tends to fracture because of internal liquid tension build-up, causing air to be pulled into the internal, liquid portion of the just cast product, leaving highly undesirable surface-connected porosity.
In lost foam casting methods, a foam pattern and gating system is ablated during filling and the heat from the casting must effect such ablation. U.S. Pat. No. 6,833,580 describes an apparatus and improved method for lost foam casting of metal articles using external pressure. That patent is hereby incorporated by reference. U.S. Pat. No. 6,883,580 allows for the application of super-atmospheric isostatic pressure onto a lost foam casting to reduce the hydrostatic tension in the molten alloy that produces undesirable surface porosity that forms if the hydrostatic tension in the molten alloy is not lowered.
However, even when AA 356 is cast using the advantageous method of U.S. Pat. No. 6,883,580, a substantial amount of misruns occur because of the relatively low heat of fusion of AA 356 to combat premature freezing from heat extraction during ablation of the foam pattern and gating system. Further, products cast with AA 356 have a significant amount of surface porosity and require restoration by spray welding. Accordingly, a better lost foam casting AlSi alloy is sought.